The neurodevelopmental, and neurocognitive deficits of HIV infection of the central nervous system (CNS) are not explainable by direct virus infection. Direct cytotoxic effects of viral gp120 on neurons, perhaps mediated through inhibition of neurotrophic growth factors, has been demonstrated. The measurement of this viral protein activity has relevance, therefore to determine the effects of HIV infection on the CNS and pediatric development. Levels of gp120 neurocytoxicity will be determined for plasma and some CSF samples from HIV pediatric patients undergoing clinical trials for neurological end points.Base line samples collected before the initiation of the trial will be tested first to determine if it is practical to measure plasma gp120 neurotoxicity levels at different stages of the disease. If so, samples collected at the end of the trial may then be tested to evaluate the ability of the assay system to monitor the efficacy of therapy. Other retroviral markers will be included to determine their comparative usefulness to monitor or predict clinical change. The measurement of biologically active viral gp120 in pediatric patient fluids will be correlated with relevant and specific neurodevelopmental and neurocognitive clinical endpoints in order to determine if viral neurocytotoxic gp120 levels correlate with symptomatology. Enhanced diagnosis, prognosis, and treatment is envisioned.